Saturday, 31 July 2010

They say that il n'y a que Paris. This is roughly true, however Paris last week was not the best place in France to learn about the latest dark matter news. Simultaneously to ICHEP'10 in Paris down south in Montpellier there was the IDM conference where most of the dark matter community was present. One especially interesting result presented there concerns the hunt for light dark matter particles.

Some time ago the CoGeNT experiment noted that the events observed in their detector are consistent with scattering of dark matter particles of mass 5-10 GeV. Although CoGeNT could not exclude that they are background, the dark matter interpretation was tantalizing because the same dark matter particle can also fit (with a bit of stretching) the DAMA modulation signal and the oxygen band excess from CRESST.

The possibility that dark matter particles could be so light caught experimenters with their trousers down. Most current experiments are designed to achieve the best sensitivity in the 100 GeV - 1 TeV ballpark, because of prejudices (weak scale supersymmetry) and some theoretical arguments (the WIMP miracle), even though certain theoretical frameworks (e.g asymmetric dark matter) predict dark matter sitting at a few GeV. In the low mass region the sensitivity of current techniques rapidly decreases. For example, experiments with xenon targets detect scintillation (S1) and ionization (S2) signals generated by particles scattering in a detector. Measuring both S1 and S2 ensures very good background rejection, however the scintillation signal is the main showstopper to lowering the detection threshold. Light dark matter particles can give only a tiny push to much heavier xenon atoms, and the experiment is able to collect only a few, if any, resulting scintillation photons. On top of that, the precise number of photons produced at low recoils (described by the notorious Leff parameter) is poorly known, and the subject is currently fiercely debated with knives, guns, and replies-to-comments-on-rebuttals.

It turns out that this debate may soon be obsolete. Peter Sorensen in his talk at IDM argues that xenon experiments can be far more sensitive to light dark matter than previously thought. The idea is to drop the S1 discrimination, and use only the ionization signal. This allows one to lower the detection threshold down to ~1 keVr (while it's order 10 times higher when S1 is include) and gain sensitivity to light dark matter. Of course, dropping S1 also increases background. Nevertheless, thanks to self-shielding, the number of events in the center of the detector (blue triangles on the plot above) is small enough to allow for setting strong limits. Indeed, using just 12.5 day of aged Xenon10 data a preliminary analysis shows that one can improve on existing limits for the dark-matter-nucleon scattering cross section in the 5-10 GeV mass interval:Most interestingly, the region explaining the CoGeNT signal (within red boundaries) seems by far excluded. Hopefully, the bigger and more powerful Xenon100 experiment will soon be able to set even more stringent limits. Unless, of course, they will find something there...

Monday, 26 July 2010

Finally, the picture we were dying to see:Tevatron now excludes the standard model Higgs for masses between 156 and 175 GeV. The exclusion window widened considerably since the last combination. Together with the input from direct Higgs searches at LEP and from electroweak precision observables it means that Higgs is most likely hiding somewhere between 115 and 155 GeV (assuming Higgs exists and has standard model properties). We'll get you bastard, sooner or later.

One interesting detail: Tevatron can now exclude a very light standard model Higgs, below 110 GeV. Just in case LEP screwed ;-) Hopefully, Tevatron will soon start tightening the window from the low mass side.
Another potentially interesting detail: there is some excess of events in the $b \bar b$ channel where a light Higgs could possibly show up. The distribution of the signal-to-background likelihood variable (which is some inexplicably complicated function that mortals cannot interpret) has 5 events in one of the higher s/b bins, whereas only 0.8 are expected. This cannot be readily interpreted as the standard model Higgs signal, as this should also produce events with higher s/b where there is none. Most likely the excess is a fluke, or maybe some problem with background modeling. But it could also be an indication that something weird is going on that does not quite fit the standard model Higgs paradigm. Maybe the upcoming Tevatron publications will provide us with more information.

Sunday, 25 July 2010

This Monday at ICHEP there will be a plenary talk by Nicolas Sarkozy. Like all theorists I'm looking forward to it, as he knows about models much more than we do. You can watch the webcast here, at high noon Paris time.

Saturday, 24 July 2010

This year CP violation in the Bs meson system has made the news, including BBC News and American Gardener. The D0 measurement of the same-sign dimuon asymmetry in B decays got by far the largest publicity. Recall that Tevatron's D0 reported 1 percent asymmetry at the 3.1 sigma confidence level, whereas the standard model predicts a much smaller value. The results suggests a new source of CP violation, perhaps new heavy particles that we could later discover at the LHC.

The dimuon asymmetry is not the only observable sensitive to CP violation in the Bs system. Another accessible observable is the CP violating phase in time-dependent Bs decays into the J/ψ φ final state. In principle, the dimuons and J/ψ φ are 2 different measurements that do not have to be correlated. But there are theoretical arguments (though not completely bullet-proof) that a large deviation from the standard model in one should imply a large deviation in the other. This is the case, in particular, if new physics enters via a phase in the dispersive part of the Bs-Bsbar mixing amplitude ($M_{12}$, as opposed to the absorptive part $\Gamma_{12}$), which is theoretically expected if the new particles contributing to that amplitude are heavy. The previous, 2-years old combination of the CDF and D0 measurements displayed an intriguing 2.1 sigma discrepancy with the standard model. CDF updated their result 2 months ago and, disappointingly, the new results is perfectly consistent with the standard model. D0 revealed their update today in an overcrowded room at ICHEP. Here is their new fit to the CP violating phase vs. the width difference of the 2 Bs mass eigenstatesBasically, D0 sees the same 1.5 sigmish discrepancy with the standard model as before. Despite 2 times larger statistics, the discrepancy is neither going away nor decreasing, leaving us children in the dark. Time will tell whether D0 found hints of new sources of CP violation in nature,
or merely hints of complicated systematical effects in their detector.

Friday, 23 July 2010

Today at ICHEP CMS and ATLAS showed their first top candidate events. They see events in both semileptonic and dileptonic channels, with muons and electrons in all combination. Here is one event display in the mu+jets+missing energy channel provided by CMS:
The reconstructed top mass from this event is around 210 GeV, while the latest measurement of the top quark mass from the Tevatron is 173.1 GeV. This is very surprising - naively, one would expect the American top quarks to be heavier ;-)

Wednesday, 21 July 2010

ICHEP'10 is starting tomorrow in Paris. As I told you the other day, I was hired to blog on the highlights of the conference. So for the entire next week I'm planning to scribble a couple of posts per day - an unusual and probably lethal frequency for a lazy blogger accustomed to writing once a month. I guess I will copy&paste the most interesting posts here to Resonaances, but if you're interested in my entire discography you should check out the official ICHEP blog. A bunch of other good fellows writing there, so should be fun.

Friday, 16 July 2010

The measurement of the Lamb shift in the muonic hydrogen has echoed on blogs and elsewhere. Briefly, an experiment at the Paul Scherrer Institute (PSI) measured the energy difference between 2S(1/2) and 2P(3/2) energy levels of an atom consisting of a muon orbiting a proton. Originally, this excercise was intended as a precise determination of the charge radius (that is the size) of the proton: in the muonic hydrogen the finite proton size effect can shift certain energy levels by order one percent, much more than in the ordinary hydrogen, while other contributions to the energy levels are quite precisely known from theory. Indeed, the PSI measurement of the proton charge radius is 10 times more precise than previous measurements based on the Lamb shift in the ordinary hydrogen and on low-energy electron-proton scattering data. Intriguingly, the new result is inconsistent with the previous average at the 5 sigma level.

As usual, when an experimental result is inconsistent with the standard model prediction the most likely explanation is an experimental error or a wrong theoretical calculation. In this particular case the previous experimental data on the proton charge radius do not seem to be rock-solid, at least to a casual observer. For example, if the charge radius is extracted from electron–proton scattering the discrepancy with the PSI measurement becomes only 3.1 sigma;
the PSI paper also quotes another recent measurement that is completely consistent with their result within error bars.

In any case, whenever a discrepancy with the standard model pops up, particle theorists cannot help thinking about new physics explanations. Our folk is notorious for ambulance chasing, but actually this is one of these cases when the ambulance is coming straight at us. Recently the particle community has invested a lot of interest in studies of light, hidden particles very weakly coupled to the ordinary matter. One example is the so-called dark photon: an MeV-GeV mass particle with milli-charge couplings to electrons and muons. This idea is pretty old, but in the past 2 years the interest in dark photons was boosted because their existence could explain certain astrophysical anomalies (Pamela). The signals of dark photons and other hidden particles are now being searched for at the Tevatron, LHC, B-factories, and in dedicated experiments such as ALPS at DESY, or APEX that is just kicking off at JLAB. No signal has been found in these experiments yet, but there is still a lot of room for the dark photon as long as its coupling to electrons and muons is $\epsilon \leq 10^{-3}$ smaller than that of the ordinary photon, see the picture borrowed from this paper. The news of the muonic Lamb shift came somewhat unexpectedly...but not to everyone: here is a passage from a 2-years old paper:

For example, the dark photon contribution to the electron-proton scattering amplitude at low momenta is equivalent to the $6 \epsilon^2 /m_A^2$ correction to the proton charge radius (...) It remains to be seen whether other precision QED tests (e.g. involving muonic atoms) would be able to improve on the current constraints.

So here we are. In the coming weeks we should see whether there exist concrete models capable of fitting all data. In any case, a new front in the battle against dark forces has just been opened. Now, could someone make us a muonium?

About Résonaances

Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The main goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...